Thermoplastic polyolefins (such as polyethylene and polypropylene) are ubiquitous items of commerce. Large volumes of these thermoplastics are extruded into sheets and films. Polyolefin films are widely used for packaging a wide variety of goods.
Thermoplastic polymers of vinyl aromatic monomers (such as styrene and alpha methyl styrene) are also well known. Such thermoplastic polymers (also sometimes referred to herein as thermoplastic styrenic polymers) are commonly used to provide rigid, clear packages for foods such as bakery goods, or opaque thermoformed containers for food and drink. Foamed polystyrene is often used to produce “clam shell” packages for take-out foods and to produce impact-resistant packages for eggs. However, polystyrene film is not in widespread use as a packaging material. Polystyrene tends to become highly oriented when extruded into thin films. This orientation may be used to generate a “splitty” film (i.e. a film with poor tear strength in the “machine direction”) or a film with predictable shrink behavior.
It is known in the art to prepare co-extruded structures having a thermoplastic polyolefin layer and a thermoplastic polystyrene layer. However, it will be recognized by those skilled in the art that thermoplastic polyolefins and thermoplastic styrenic polymers have different polarities. Accordingly, it is difficult to produce such coextruded structures with layers which adhere to one another. The prior art discloses several attempts to mitigate this adhesion problem, as briefly set out below.
U.S. Pat. No. 4,626,455 (Karabedian, to Owens-Illinois) teaches a two layer coextruded structure in which a foamed polystyrene layer is adhered to a polyolefin layer. The polyolefin layer also contains polystyrene and a compatibility agent which is preferably a styrene-butadiene block copolymer.
U.S. Pat. No. 4,440,824 (Bonis, to Composite Container, Inc.) teaches a multilayer coextruded structure comprising a polyolefin layer; a high impact polystyrene layer; and an adhesive tie layer which is prepared from either ethylene-vinyl acetate copolymer or an ethylene-acrylic acid copolymer.
U.S. Pat. No. 5,219,666 (Schirmer et al., to W. R. Grace & Co.) teaches a five layer film in which two outer (skin) layers of styrene butadiene copolymer are bonded to a core layer of very low density polyethylene using two ethylene-vinyl acetate tie layers (with each tie layer being located between the core layer and the two skin layers).
U.S. Pat. No. 4,879,177 (Boice, to W. R. Grace & Co.) also teaches a five layer film in which a core layer (which may be a butadiene styrene copolymer) is sandwiched between two ethylene copolymer tie layers. The outer or skin layers of this structure are ethylene propylene copolymers, polypropylene or blends thereof.